The present invention relates to a method of in situ retorting of oil shale. The invention further relates to the in situ retorting of oil shale using a particulate material, preferably a cracking catalyst, in combination with a gaseous flow of oxidizing material.
Explosives are normally utilized to rubbilize the shale. Pillars and open chambers are coated and then dynamited to form a rubberized chamber of shale rock. This cavern is surrounded by a thick zone of fractured, but nevertheless impervious, shale or rock. The fractured rock, collapses so as to at least partially fill the cavity. Thus, a substantially cylindrical retort zone or chimney filled with crumbled oil shale results. This also leaves an empty area or void at the uppermost portion of the chimney. Such techniques are described, for example, in U.S. Pat. No. 3,342,257, issued Sept. 19, 1967 to Jacobs et al, and in U.S. Pat. No. 3,113,620, issued May 2, 1978 to Hemminger.
There are several problems encountered with this method of in situ retorting of oil shale. The combustion zone or area is continuously advancing through the chimney. In order to efficiently retort most of the oil within the chimney, however, the combustion zone must advance uniformly through the chimney at a relatively slow rate. If the combustion zone moves too quickly, channeling will tend to occur. Channeling is the movement of the retort gases along a channel as opposed to moving uniformly through the shale. If the combustion zone moves erratically, various parts of the chimney will not be retorted and the oil contained in these portions will be lost.
One of the causes of channeling and erratic movement of the combustion zone is improper rubblization in localized areas. If the rocks or shale are not properly rubblized, or are not broken into small enough fragments, the retort reaction will tend to occur on the surface of the shale and will not penetrate rock fragments and draw out the oil within the inner portion of an oil shale rock. It is extremely difficult to provide uniform rubblization. The erratic stratification of the shale formations, as well as various other factors, also prevents more uniform rubblization.
Even when the combustion zone moves at a desired rate, the produced oil tends to be of a low grade, high molecular weight oil requiring substantial additional processing. The produced oil tends to be high in nitrogen, sulfur, and arsenic, which must be removed.
Accordingly, it is an object of the present invention to improve the efficiency of in situ oil shale conversion, to provide a higher percentage of recovery of the oil, and to produce liquid products of a higher grade than ordinarily produced, with lower levels of impurities.
Furthermore, it is an object of the present invention to provide a method to overcome channeling and provide for substantially complete retorting of the oil shale within a chimney.